1. Field of the Invention
The present invention relates to an engine system, a method for controlling an engine system, and a vehicle including the engine system.
2. Discussion of the Background
FIG. 13 is a graph showing constant fuel-consumption contours and an optimum-fuel-consumption line of an engine of a vehicle. In FIG. 13, an area A corresponds to the state in which the vehicle is driven in an urban area. In the area A, a throttle opening is small and the engine efficiency is low. An area B corresponds to the state in which the vehicle is accelerating, and an area C corresponds to the state in which the vehicle is moving at high velocity.
In a hybrid vehicle, when the desired engine output is in the area A, the engine is actually operated in the area B due to an increase in the load to generate electric power. Alternatively, the engine is stopped and the vehicle is driven only by an electric motor. Thus, the engine is prevented from operating in the area A, where the engine efficiency is low.
In the case in which the hybrid vehicle is driven only by the electric motor in the area A, the electric motor is required a rated output of about one half of the vehicle's maximum output and a battery is required to be large enough to drive the vehicle in the area A by the electric motor. When such an electric motor is installed in the hybrid vehicle, the volume, weight, and cost of the vehicle increase.
In addition, in the hybrid vehicle, the generated electricity is first stored in the battery and the electric motor is driven by the electricity stored in the battery. Therefore, considering the efficiency of the electrical system (=generating efficiency×charging efficiency×discharging efficiency×driving efficiency of the electric motor), there may be a case in which the fuel consumption cannot be sufficiently reduced even when the vehicle is driven by the electric motor in the area A.
In addition, in the hybrid vehicle, the maximum engine output is reduced and the maximum output is obtained using both the engine and the electric motor. Therefore, when the hybrid vehicle is constantly driven under high-load conditions (for example, when the vehicle is pulling heavy loads or cruising at high velocity on a freeway), the vehicle will eventually be driven only by the engine because of the insufficient battery capacity. Accordingly, sufficient engine power cannot be obtained.
As one of the solutions for the above-described problems, a technique regarding a cylinder number variable engine in which the number of activated cylinders can be controlled has been suggested in, for example, Japanese Unexamined Patent Application Publication No. 2002-13423. The contents of this application are incorporated herein by reference in their entirety.
FIG. 14A is a graph showing an optimum-fuel-consumption line when a cylinder number variable engine performs an all-cylinder operation in which all of the cylinders are activated. FIG. 14B is a graph showing an optimum-fuel-consumption line when the cylinder number variable engine performs a reduced-cylinder operation in which some of all cylinders are activated. According to the technique described in Japanese Unexamined Patent Application Publication No. 2002-13423, the cylinder load is increased by causing the cylinder number variable engine to perform the reduced-cylinder operation, so that the operation of the area A would be performed in the area B, as shown in FIG. 14B, and the fuel consumption rate is improved. In addition, the all-cylinder operation is performed in the high-load areas (areas B and C) so that high-load operation can be performed continuously.
Japanese Unexamined Patent Application Publication No. 57-176330 discloses a technique for reducing the torque gap when the number of cylinders activated in the cylinder number variable engine is changed. The contents of this application are incorporated herein by reference in their entirety.
FIG. 15 is a graph of output torque versus throttle-valve opening in a 4-cylinder operation and a 2-cylinder operation. The engine speed is constant. According to the technique described in Japanese Unexamined Patent Application Publication No. 57-176330, the number of cylinders is changed at a throttle-valve opening corresponding to the intersection of the engine-torque lines corresponding to the all-cylinder operation and the reduced-cylinder operation, as shown in FIG. 15. Many cylinder number variable engines in practical use adopt this method for changing the number of cylinders.
Japanese Unexamined Patent Application Publication No. 7-293288 discloses a cylinder number variable engine in which the point at which the number of cylinders can be changed is not limited so that the effect of reducing fuel cost is enhanced. The contents of this application are incorporated herein by reference in their entirety.
According to the technique described in Japanese Unexamined Patent Application Publication No. 7-293288, a group of continuously activated cylinders and a group of cylinders which are stopped as necessary are provided with respective throttle valves, and the two throttle valves are operated in association with each other such that the torque gap does not occur when the number of cylinders is changed.